1.1 Field of the Invention
The invention pertains to a specific design of a biological fluidized bed apparatus to treat wastewater. The invention claimed here is the particular design of the apparatus and its components.
1.2 Description of the Prior Art
Over the last twenty years, extensive research has been done in the US, Europe and Japan to develop various fluidized bed reactor configurations and processes. Important patented contributions in this field have been made by the following:
Rovel et al. (pat: U.S. Pat. No. 4,482,458)
Vogelpohl et al. (pat: U.S. Pat. No. 4,940,546)
Edwards (pat: U.S. Pat. No. 5,441,634)
Yoda et al. (pat: U.S. Pat. No. 4,762,612)
Clark et al. (pat: U.S. Pat. No. 5,942,116)
Love (pat: U.S. Pat. No. 4,530,762)
Klein (pat: U.S. Pat. No. 5,573,671)
Biological fluidized bed reactors have been widely used to remove dissolved and suspended organic matter from high-strength industrial effluents. In this application, the biological beds are comprised of anaerobic bacteria. Anoxic fluidized bed reactors have also been used for the removal of nitrate from industrial and municipal effluents, in which case the biological bed is comprised of denitrifying bacteria.
In the anaerobic application, these reactors convert dissolved and suspended organic matter into methane and carbon dioxide (biogas). The conversion is accomplished by anaerobic bacteria, which grow attached as biofilm to inert media particles in the fluidized bed. The reduction of organic matter in the treated waste is the result of a combination of physical retention of suspended and colloidal organic matter by solids contact flocculation within the biological bed, hydrolysis of the trapped solids by hydrolyzing bacteria present in the anaerobic consortium, and finally biological conversion of dissolved organic matter into biogas by acetogenic and then methanogenic bacteria.
In the anoxic application, these reactors convert nitrate into nitrogen gas. The conversion of nitrate to nitrogen gas is accomplished by denitrifying bacteria that grow attached to inert media particles in the fluidized bed. These heterotrophic bacteria are ubiquitous in most natural waters. In the process of denitrification, nitrate acts as an electron acceptor and an organic substrate as a reduced electron donor. The substrate used as an electron donor and a source of carbon is a readily degradable organic substrate (i.e., methanol, sugars or acetic acid), which is supplemented to the anoxic reactor.
The fluidized bed reactors in use are an attached biofilm fluidized bed process that is based on the concept that large biomass concentrations can be achieved on a large surface area by dense biofilm attachment to an inert small particle size carrier. A large surface area is created by small inert particles in a bed, fluidized by upward flow. The intense mixing occurring in the bed minimizes diffusion limitations and eliminates clogging and short-circuiting problems. These reactors accumulate large amounts of active biomass, and can achieve very long cell detention times. Existing fluidized bed reactors consist of a cylindrical column with flat bottom. These columns contain a bed of small inert particles, 0.4-1.0 mm zeolite, diatomaceous earth, or activated carbon particles. The bed is fluidized by the upward flow through the column of untreated wastewater mixed with recirculated effluent. In existing reactors, the upward flow necessary to achieve bed fluidization is distributed by means of various nozzles or small diameter pipes placed at the bottom of the column. Anaerobic biofilm develops on the inert media and the physical attachment of anaerobic bacteria to the media surface prevents biomass washout. The high fluid shear force resistance of biofilms allows these reactors to be operated at upflow velocities which would otherwise wash out unattached biomass.
Upflow velocities are set by the recirculation flow, according to the fluidization properties of the bio-particles (e.g., inert particles with attached bacterial film). The upflow velocities are sufficient to reach bed expansions as to allow free release of generated gas bubbles. The degree of bed expansion is achieved by controlling the recirculation flow rate of a portion of the effluent in a closed-loop.
Under some conditions the turbulent flow exerts sufficient shear to prevent the development of thick biofilms on the media, which limit mass transfer. The high surface-to-volume ratio of the bulk of the bed inert media (300 to 2500 ft2/ft3) creates a vast area for the development of microbial biofilm. Approximately 95 percent of the active biomass in a well-operated fluidized bed reactor is attached growth. This fact enables the development of dense but thin biofilms that lead to high concentrations of attached biomass in the bed.
Despite the great advantages that this process offers in terms of high organic loading rates, short hydraulic retention times and low excess solids generation, the use of these reactors has not been extensive due to various design limitations. The most common problems reported in full-scale applications and their effects on operation and effluent quality are:
(a) Inadequate Flow Distribution at the Bottom of the Bed
In current fluidized bed reactor configurations, clogging of distribution nozzles and the existence of xe2x80x9cdead zones,xe2x80x9d channeling and short-circuiting inside the bed are major disadvantages. Good flow distribution is necessary to achieve uniform and controlled bed expansion and a well-mixed flow pattern inside the bed. Such patterns promote turbulence at the biofilm/liquid interface and enable all the attached biomass to be in contact with the waste.
When a uniform expansion of the bed is obtained, biogas bubbles generated in the bed are evenly released. This avoids the coalescence of small bubbles into much larger bubbles that disrupt the bed as they rise. Such bed disruptions deteriorate the quality of the effluent by releasing solids trapped within the bed.
Although the solution to inadequate bed expansion would appear to be increasing the recirculation flow in order to achieve higher upflow velocity, this presents the disadvantage of washing solids trapped in the bed, by the high interstitial velocities created by the increased flow. These solids deteriorate the quality of the effluent.
(b) Need for Highly Uniform Particle Size Media in Cylindrical Reactor Configurations
In cylindrical, flat bottom reactors, the inert media must have a highly uniform particle size. Typical media materials are zeolite, sand, and activated carbon. Commercially available media are not highly uniform in particle size. A more uniform particle size media has a higher cost, since narrow particle size range sieving produces more wasted material in the for the media manufacturer.
Since the upflow velocity in cylindrical reactors is constant throughout the bed, the existence of various particle sizes affects uniform expansion. Larger particles weigh more and have higher terminal settling velocities, thus, higher upflow velocities are required to keep them suspended. If there is a range in the particle size of the media, at the upflow velocity necessary to expand the large particles, the small particles are carried out of the bed, or over expanded. On the other hand, at an upflow velocity, at which small media particles reach adequate fluidization, large particles remain unexpanded at the bottom of the reactor, creating a plug and hindering uniform expansion.
(c) Inadequate Solids/Gas/Liquid Separation Within the Reactor
Before the treated effluent exits the top of the reactor, suspended solids need to be removed. The rising bubbles above the bed create a drag effect that helps carry suspended solids to the top of the reactor. Gas bubbles also tend to trap suspended solids, which are attracted by the surface tension of the bubbles. In several reactors without an appropriate incorporated solids separation system organic loadings are kept below the reactor""s capacity in order to reduce gas generation as a measure to limit suspended solids concentrations in the effluent. In several reactor configurations, additional equipment such as external clarifiers is used to polish the effluent.
Many industrial and farm wastes produce a scum layer at the water surface inside the reactor. This layer of floating grease and organic material needs to be removed so that it does not accumulate or appear in the effluent. To the extent of our search and experience, no current biological fluidized bed reactor in use, presents a surface skimmer and an effluent launderer to achieve this objective.
(d) Difficulty in Removing Excess Solids from the Reactor
Anaerobic bacteria are slow growers, however as a result of substrate consumption there is biomass growth. Thus, there is a need to accumulate and store detached biomass inside the reactor and to remove it for disposal. It is important to provide sufficient solids retention time inside anaerobic reactors, in order to achieve adequate digestion of the excess solids produced. This eliminates the need for further sludge stabilization outside the reactor. However, failure to accumulate and remove excess biomass without disturbing the operation of the reactor, will affect the performance of the unit and the quality of the effluent.
This biological fluidized bed apparatus provides a pre-fabricated, modular, self-contained biological reactor, which can treat municipal, industrial and confined animal feedlot wastewater at high loading rates. This reactor design overcomes the limitations and disadvantages of prior fluidized bed reactor configurations by means of its shape and internal components. As a result, it provides better quality effluent, requires less xe2x80x9cfoot printxe2x80x9d area for installation and demands less maintenance.
This said apparatus overcomes several disadvantages of current fluidized bed reactor configurations. The improvements in this apparatus"" design are:
A. Uniform flow distribution at the bottom of the bed. This is achieved using a reactor tank with a 60-degree conical bottom (inverted truncated cone) and a single internal down-coming pipe discharging recirculated flow at the bottom of the cone. In order to limit turbulence at the bottom of the reactor, the flow exits the pipe radially by means of a molded flow distribution fitting. This fitting, which changes the direction of the flow from vertical to radial, is contoured internally to reduce losses and smoothen the change of direction of the flow lines. The open area of the flow distribution fitting is such that exit velocities are sufficiently high to drag the larger bioparticles located at the bottom of the reactor. The required drag velocity is achieved at a flow sufficient to create the upflow velocity required for the minimum admissible bed expansion. The proposed flow distribution design eliminates nozzle clogging, since a single fitting is used instead of a network of multiple small diameter nozzles. With this design, no channeling or xe2x80x9cdead zonesxe2x80x9d form in the bed. Additionally, bed expansion is uniform and can be controlled very precisely by adjusting the recirculation flow. A more uniform fluidization enables gas bubbles to be released evenly throughout the section of the bed.
B. Media with a lower particle size uniformity coefficient can be efficiently fluidized. The conical bottom reactor design allows a wider range of particle sizes in the media to be fluidized, as a result of the various upflow velocities occurring through the conical portion of the reactor. At a constant recirculation flow, upflow velocities gradually decrease from the bottom of the inverted cone through the cylindrical portion of the tank. This is a result of the change in sectional area of the conical portion. Having various upflow velocities in the lower section of the reactor accommodates the use of a wider range of particle sizes in the media. Larger, heavier particles will concentrate towards the bottom of the cone, where higher upflow velocities occur due to reduced cross-sectional area. Accordingly, smaller, lighter particles will accumulate in the upper section of the cone, which has lower upflow velocities. This feature enables the use of biofilm support media with less uniform particle size. This reduces the cost of the media since it can be purchased as a readily available commercial product.
C. Solids carry-over with the reactor effluent greatly minimized. The apparatus configuration includes an internal downflow solids/gas/liquid separator at the top of the unit. Solids carried out of the bed are forced to flow downward at high velocity along the sloped wall of the separator, into a submerged trough. The upflow velocity above the trough is extremely low, such that the solids could not be picked up from the trough. A slow-rotating paddle sweeps the trough and pushes most of the solids into two collection boxes connected to the intake of the recirculation pump. This way, most of the solids retained in the separator are forced to re-enter the bed. A portion of the separated solids is pushed into other two deeper collection boxes for accumulation, thickening and periodic wasting. Thickened solids sludge is removed from these boxes by means of electrically actuated valves, operating on timers. Additionally, the reactor includes a quiescent clarification zone, external to the separator.
Gas bubbles are isolated from the clarification zone and do not interfere with this process. The clarification zone has a gradual increase in cross-sectional area; thus, it operates as an upflow solids contact clarifier. On top of the clarification zone, an effluent trough disposed on the perimeter of the reactor collects the effluent overflow. An effluent launderer, installed along the trough, retains floating solids. A slow-rotating skimmer arm connected to the same sweeping mechanism of the submerged trough, pushes scum and floating solids on the surface of the water into a scum box. Scum and floatables collected in this box are removed by means of an electrically actuated valve. This internal solids/gas/liquid separator provides a reliable means for separation of the three phases. The separator provides an enclosed volume above the surface of the water for the accumulation and removal of biogas.
The advantages noted above and other benefits of our Biological Fluidized Bed Apparatus over prior art will become apparent from a consideration of the following descriptions and drawings.
The Biological Fluidized Bed Apparatus depicted in the figures is a modular, prefabricated reactor. Projects that require larger reactor volumes should use two or more units connected in parallel or in series, as required. For lower loading requirements, smaller modular units can be fabricated, maintaining the proportion of the dimensions presented in the drawings.